The invention relates to a differential digital/analog converter and in particular to a segmented differential digital/analog converter with an integrated current matching circuit for matching the currents delivered by the segments.
Digital/analog converters are electronic circuits which convert a digital signal, i.e. a binary coded digital word, into an analog signal. The conversion takes place at discrete points in time, so that the analog signal is available at specific time intervals.
FIG. 1 shows a digital/analog converter according to the prior art. The binary-weighted current sources I0, I1, I2, . . . used by the digital/analog converter DAC are binary-weighted current sources which deliver current values I, 2I, 4I . . . . The current sources can be switched via switching devices to a current collecting line SL, which is connected to the inverting input of an operational amplifier OP. The non-inverting input of the operational amplifier OP is connected via a line to ground.
The switching devices are formed by the diodes represented in FIG. 1. If, for example, the data bit b0 has the data value 0, the diode D0b connected to the current source I0 is turned on and the current source I0 is bridged. If, conversely, the data bit b0 is equal to 1, the diode D0b and the current source I0 will provide a contribution to the current on the current collecting line SL. The output of the operational amplifier OP is fed back to the inverting input via a feedback resistor RN. The analog output voltage UDAC present at the output of the digital/analog converter DAC is proportional to the sum of the currents switched through to the current collecting line SL and is consequently proportional to the applied digital value. The following holds true:
Ii=2ixe2x88x921xc2x7ILSB
However, the conventional digital/analog converter represented in FIG. 1 has the disadvantage that it is not of a differential construction, but merely supplies an analog output voltage UDAC to ground.
In the case of an integration of the conventional DAC shown in FIG. 1, the analog output signal delivered by the DAC is susceptible to interference signals, which originate from a common substrate on which the DAC and further circuits are located.
A further disadvantage of the conventional prior-art DAC represented in FIG. 1 is that a circuit arrangement with further circuits for signal processing of the analog signal delivered is possible only with additional circuitry, since a common mode recovery is necessary.
It is therefore an object of the present invention to provide a digital/analog converter which is insensitive to interference signals from further circuits.
This object is achieved according to the invention by a digital/analog converter of a fully differential construction with the features specified in patent claim 1.
One advantage of the digital/analog converter according to the invention is that it can be connected without additional circuitry to other circuits, which carry out subsequent analog processing of the analog output signal.
The invention provides a differential digital/analog converter for the conversion of a digital input value into an analog output voltage with
a first current source group, which has a plurality of current sources comprising at least one p-MOS transistor, which can be switched to at least one current collecting line in dependence on the applied digital input value,
a second current source group, which has a plurality of current sources comprising at least one n-MOS transistor, which can be switched to the current collecting line in dependence on the applied digital input value,
and with an output buffer for the conversion of the current flowing on the current collecting line into the analog output voltage.
In the case of a preferred embodiment of the differential digital/analog converter according to the invention, the first and second current source groups in each case have a thermometer current source switching segment, comprising thermometer current sources, and a binary current source switching segment, comprising binary current sources.
In the case of a further preferred embodiment of the differential digital/analog converter according to the invention, the digital/analog converter has a current matching circuit, which is provided for matching that current which is delivered by the binary current source switching element of the first current source group to that current which is delivered by the thermometer current source switching segment of the second current source group
and which is provided for matching that current which is delivered by the binary current source switching segment of the second current source group to that current which is delivered by the thermometer current source switching segment of the first current source group.
This offers the particular advantage that linearity errors caused by the segmentation are avoided.
The digital input value to be converted, present at an input, preferably comprises k data bits, the n least significant data bits of the digital input value switching the two binary current source segments respectively comprising n binary current sources to the current collecting line,
the m most significant data bits being applied to a decoder for switching 2mxe2x88x921 thermometer current sources, which are in each case provided within the two thermometer current source switching segments, to the current collecting line.
The binary current source switching segments preferably comprise in each case n binary current sources, the current Ii delivered by the ith current source (1xe2x89xa6Ixe2x89xa6n) amounting to
Ii=2ixe2x88x921xc2x7ILSB,
where ILSB is the current which is delivered by that current source which is assigned to the least significant data bit of the digital input value.
The output buffer preferably has a fed-back operational amplifier.
In this case, the operational amplifier is preferably fed back via at least one resistor.
In the case of a particularly preferred embodiment of the differential digital/analog converter according to the invention, the current matching circuit is a current mirror circuit, which contains at least one current mirror transistor for matching the current which is delivered by the binary current source segments to that current which is delivered by the thermometer current source segments.
The mirror current generated by the current mirror transistor preferably corresponds to the sum of the current delivered by the binary current source switching segment and the current of the binary current source for the least significant data bit.